Modern network structures are typically comprised of a network of networks. Large, geographically dispersed wide area networks (WANs) connect numerous metro area networks (MANs) which in turn connect the local area networks (LANs) originating from residences and businesses. Most network data (non-voice data), originates from LANs and is destined to other LANs. WANs are essentially the highways of modern communications networks, transmitting very large amounts of data traffic at very high speed where that data information is then exited at the MANs and routed to the appropriate address in the appropriate LAN. In order to keep up with the ever increasing bandwidth needs, modern WANs frequently transmit data optically between long distances, or wherever particularly high bandwidth connections are needed, using synchronous optical networking (SONET) rings. SONET rings can be viewed as large rings with nodes distributed at points along the ring. Data traffic, in the form of optical pulses circles the ring along a given fiber. If the data needs to be switched from one fiber to a next, or exited from the ring and routed to its destination, the optical signal must be converted to an electronic signal and then either sent over an electronic network to its destination or reconverted to an optical signal for further transmission on the same or an alternate SONET ring/fiber.
Although today's optical networks, which are primarily confined to the Internet backbone WANs, are actually hybrid opto-electronic networks, it is envisioned that future communications networks will be designed and built from all optical networks extending from LANs all the way to the backbone WANs. In an all optical network, there will be no conversion of optical signals to electronic signals except at the source and destination of the data. All the switching between fibers, routing of data and signal amplification will be accomplished optically. Furthermore, because signal switching, routing and amplification in a communications network is in principal no different than signal switching, routing and amplification in a computer circuit, the same underlying principles and components used in optical networking will ultimately be the basis for optical computers, creating the next great information revolution.
To fully exploit the potential of fiber optic communications, integrated optic components that function at faster transmission rates and higher bandwidths are essential. The low transmission losses, immunity to electrical noise, and huge bandwidth capabilities of optical components are also factors fueling research to develop integrated optic devices—the optical counterpart of integrated electronics. Optical integration and miniaturization also imply increased signal speed, lower optical loss because of reduced optical pathlengths, reduced power requirements, flexibility, ruggedness, compactness, and potential low cost.
Although integrated optics technology is promising, the commercial realization of the promise of integrated optics technology is directly dependent on the establishment of cost effective, reliable techniques of fabricating passive and active waveguiding structures. To date, neither of these requirements has been satisfactorily met. Nor are the current technologies sufficiently flexible.
Optical communications hardware is extraordinarily expensive to build because unlike electronic communications hardware, which is largely implemented with modern integrated circuitry, optical communications equipment is typically comprised of discrete optical components. For example in an electronic multiplexing circuit, the filters, amplifiers and conducting paths are all integrated into a single chip manufactured in one process, but in its optical analog, a wavelength division multiplexer, the analogous filters, amplifiers and waveguides are typically discrete components, individually connected and mostly by hand.
Under currently known technologies, it is even complicated to fabricate channel waveguides—the basic building block structures for integrated optic devices and components—that are suitable for current optical communication applications. For compatibility with optical fibers, channel waveguides are frequently buried in material having a lower index of refraction. This is typically accomplished by first constructing a core layer of a doped SiO2 layer, and then etching the core layer into a rectangular cross-section to form a ridge or strip waveguide. A cladding layer is then used to bury the waveguide cores, as well as to symmetrise the modal fields and isolate them from their surroundings. Thus, waveguides are conventionally fabricated out of glass by multi-step processes involving a combination of lithographic photomask technologies and etching processes. However, the equipment historically used for such etching processes, e.g., reactive ion etchers, has required substantial capital investment.
Technologies available for fabricating the glass layer necessary for producing integrated optics devices currently include: sputtering, thermal oxidation and nitridation, chemical vapor deposition, plasma-enhanced chemical vapor deposition, flame hydrolysis deposition, and sol-gel deposition. The first two technologies are too restrictive, and only the last four are currently under investigation. These different technologies have been discussed in R. R. A. Syms, Advances in Integrated Optics, Chapter 7, Silica-on Silicon Integrated Optics, pp. 121-150 (1994).
Another problem with the majority of the known technologies is that they involve high temperature processing (>1000° C.), which can be incompatible with constructing a hybrid opto-electronic device on a semiconductor substrate.
While plasma enhanced chemical vapor deposition is a low temperature process, this technology entails complex multistep mask processes including reactive ion etching.
Conventional sol-gel derived glass technologies have required high temperature post-film deposition thermal treatments. In addition, etching and multistep masking have been required to complete the integrated optics device. Further, if sol-gel films greater than about 1 to 2 μm are desired, multiple coating steps with a drying step between each has been required to prevent cracking during consolidation of the film.
One attempt at advancing the sol-gel derived glass technology is described in Canadian Demand Application No. 2,218,273 and U.S. Pat. No. 6,054,253, which are hereby incorporated by reference. These patents describe a process for forming an integrated ridge waveguide on a silicon substrate. The disclosed process includes the steps of producing a film of photosensitive sol-gel on a substrate, applying a photomask having an opening defining the ridge waveguide, exposing the sol-gel film through the photomask to ultraviolet radiation to render the exposed portion insoluble to a given solvent, and wet etching the sol-gel film with a solvent to dissolve the unexposed portion of the sol-gel film and leave the exposed portion of the film, and therefore the ridge waveguide. The ridge waveguide is subjected to a post-baking process at a temperature of less than or equal to 200° C. to further polymerize the remaining sol-gel and drive off moisture. A cladding layer may be added on top of the ridge waveguide to bury it.
Although a step in the right direction, the foregoing process has serious deficiencies. First and foremost, absent resorting to extraordinary measures such as ion implantation, local densification of silica glass by either laser or electron beam irradiation, or multistep deposition, masking, and etch back procedures, it is not possible to vary the index of refraction in the wave guiding portion of the device. Thus, the range of optical devices that may be fabricated is very limited. To make the analogy to electronic circuitry, it would prohibit devices that require the resistance to be varied. Secondly, the etching step tends to also partially etch the sidewalls of the nascent optical device creating tapering and rough edges that result in sidewall scattering, and generally compromise the device's properties as well as the repeatability of the fabrication process. Third, even if a cladding layer is applied in an attempt to planarize the resulting device, as with other topographic waveguide fabrication techniques, it is difficult to achieve a completely planar surface in the final device.
A technique for making planar waveguides without etching is disclosed in Hensch, U.S. Pat. No. 5,080,462. The Hensch method involves local densification of a silica matrix to vary the refractive index of the silica matrix as a function of density. Hensch teaches that integrated optical devices may be formed by densifying a silica sol or monolithic silicon by heating a portion of the silica sol or the monolithic silicon relative to another portion to vary the index of refraction of the one portion relative to the other. Because of the difficulties in controlling the diffusion of heat, Hensch's techniques are not practical for the fabrication of optical devices involving complex geometries, critical feature sizes or areas of highly or tightly varied indices of refraction.
The doctoral thesis Photolithography of Integrated Optic Devices in Porous Glass, City University of New York, 1992 by E. Mendoza, one of the applicants herein, describes techniques for fabricating integrated optic devices in bulk porous glass materials. The process includes diffusing an organometallic photosensitizer into a bulk porous glass matrix so that it adsorbs on the surface of the glass, writing the image of an optical device using photolithography, and then fixing the image through two heat treatment steps. The bulk porous glass material primarily investigated in the thesis report is porous Vycor glass (PVG). However, the thesis also states that sol-gel techniques may be used to make the porous glass bulk material.
Although permitting simple devices to be produced that have a highly resolved refractive index gradient between the printed waveguide and surrounding glass, the process and devices disclosed in the thesis suffer from several important drawbacks. Because the photosensitizer is diffused into the porous glass matrix, a concentration gradient exists. This leads to variations in the refractive index of the device in the direction of diffusion. Further, as the porosity of the bulk material is not homogeneous over its entire surface, variations in the amount of photosensitizer that diffuses into the matrix over the surface of the glass material are experienced as well, which in turn leads to unwanted variations in the refractive index in the plane of the device. Because the process is diffusion based, it is also difficult to precisely control the dimensions of the resulting waveguide, particularly in the direction of diffusion.
In E. Mendoza, et al., Photolithography of Integrated Optic Devices in Sol-Gel Glasses, SPIE vol. 2288, 580-88 (1994), a process is described for making embedded channel waveguides in a sol-gel film deposited on a glass substrate. According to the described process, a sol-gel was produced using TEOS, ethanol and water in the molar proportions of 1:5:6, respectively. The solution was allowed to hydrolize and polymerize for 12 hours, at which point a photosensitive organometallic compound, dissolved in ethanol, was added to the aged solution. The photosensitized solution was then allowed to mix for 30 minutes, after which a film of the photosensitized sol-gel was formed on a glass substrate by either spinning or dipping. This solution permitted films ranging from 0.25 μm to 2.0 μm to be produced with multiple coatings being required for thicker films in the range. The article also notes that if a binder of polyethylene glycol (PEG) is added to the sol-gel solution during polymerization of the sol, films up to approximately 10 μm in thickness can be produced. However, multiple coatings with intervening drying steps were still required to produce films greater than about 1 μm without cracking. According to the article, these coatings were produced by first coating the glass substrate with the sol-gel solution in a thickness of less than 1 μm, followed by drying the film at 150° C. for 5 minutes. Once the film was dry, additional coatings were applied in the same manner to produce a photosensitized sol-gel film having the desired thickness. Immediately after being coated onto the glass substrate, the photoactive films were exposed to light, followed by a thermal curing treatment at 400° C. for 5 minutes.
While the above approach eliminates issues relating to concentration gradients resulting from diffusion, it is still deficient. First, even with the use of a binder, multiple coatings are required to produce a film of suitable thickness for most optical communication applications (e.g., approximately 6 to 10 μm). Second, the addition of PEG to the glass matrix, weakens the glass significantly because polyethylene is introduced into the glass matrix. Third, the described process simply does not work for a significant number of photosensitizers.
An object of the present inventions is to overcome, or at least ameliorate, one or more of the above-discussed drawbacks. To this end, one aspect of the present invention is directed to providing an improved process for fabricating integrated optic devices. In addition, an improved integrated optic device layer is provided that may be used to fabricate a wide variety of improved integrated optic devices.